Interesting,
Technically Hertz cant charge you if they dont have a signed VPI (inspection) document confirming no damage, but they seem to do this. It is the branch that raises the charges and thinks hey they have insurance they can cover it.
Most company's can obtain a repair estimate and estimate the charges, present it to a court and say thats what they are claiming as Cash In Leiu of repairs. Legally speaking they are entitled to do this, what you can do is dispute the costs of the estimates.
I would suggest, if this does happen to you in future get the rental closed down in front of you and the bill confirming no charges. Whilst technically they can still add in the charges later you have added weight it was checking in and confirmed as fine as you have the bill confirming no charges and check in.
Its always good to take pics but in honesty they use the signed VPI, pics can be manipulated and and the date and time etc can be edited. I always over mark the VPI with the smallest mark i see, marked numerous times rather then one big mark excpet for the alloy where i mark the entire tyre